Mutation in subdomain G' of mitochondrial elongation factor G1 is associated with combined OXPHOS deficiency in fibroblasts but not in muscle

The mitochondrial translation system is responsible for the synthesis of 13 proteins required for oxidative phosphorylation (OXPHOS), the major energy-generating process of our cells. Mitochondrial translation is controlled by various nuclear encoded proteins. In 27 patients with combined OXPHOS deficiencies, in whom complex II (the only complex that is entirely encoded by the nuclear DNA) showed normal activities, and mutations in the mitochondrial genome as well as polymerase gamma were excluded, we screened all mitochondrial translation factors for mutations. Here, we report a mutation in mitochondrial elongation factor G1 (GFM1) in a patient affected by severe, rapidly progressive mitochondrial encephalopathy. This mutation is predicted to result in an Arg250Trp substitution in subdomain G' of the elongation factor G1 protein and is presumed to hamper ribosome-dependent GTP hydrolysis. Strikingly, the decrease in enzyme activities of complex I, III and IV detected in patient fibroblasts was not found in muscle tissue. The OXPHOS system defects and the impairment in mitochondrial translation in fibroblasts were rescued by overexpressing wild-type GFM1, establishing the GFM1 defect as the cause of the fatal mitochondrial disease. Furthermore, this study evinces the importance of a thorough diagnostic biochemical analysis of both muscle tissue and fibroblasts in patients suspected to suffer from a mitochondrial disorder, as enzyme deficiencies can be selectively expressed.     

Mutation Spectrum and Genotype–Phenotype Correlation in Cornelia de Lange Syndrome

Cornelia de Lange syndrome (CdLS) is a clinically and genetically heterogeneous developmental disorder. Clinical features include growth retardation, intellectual disability, limb defects, typical facial dysmorphism, and other systemic involvement. The increased understanding of the genetic basis of CdLS has led to diagnostic improvement and expansion of the phenotype. Mutations in five genes (NIPBL, SMC1A, SMC3, RAD21, and HDAC8), all regulators or structural components of cohesin, have been identified. Approximately 60% of CdLS cases are due to NIPBL mutations, 5% caused by mutations in SMC1A, RAD21, and HDAC8 and one proband was found to carry a mutation in SMC3. To date, 311 CdLS‐causing mutations are known including missense, nonsense, small deletions and insertions, splice site mutations, and genomic rearrangements. Phenotypic variability is seen both intra‐ and intergenically. This article reviews the spectrum of CdLS mutations with a particular emphasis on their correlation to the clinical phenotype.     

Incidence and clinical features of X-linked Cornelia de Lange syndrome due to SMC1L1 mutations

Cornelia de Lange syndrome (CdLS) is a multisystem developmental disorder characterized by facial dysmorphism, growth and mental retardation, microcephaly, and various malformations. Heterozygous mutations in the NIPBL gene have been detected in approximately 45% of affected individuals. Recently, a second CdLS gene, mapping to the X chromosome, has been identified: SMC1L1 (structural maintenance of chromosomes 1-like 1; or SMC1A). In order to estimate the incidence and refine the clinical presentation of X-linked CdLS, we have screened a series of 11 CdLS boys carrying no NIPBL anomaly. We have identified two novel de novo SMC1L1 missense mutations (c.587G>A [p.Arg196His] and c.3254A>G [p.Tyr1085Cys]). Our results confirm that SMC1L1 mutations cause CdLS and support the view that SMC1L1 accounts for a significant fraction of boys with unexplained CdLS. Furthermore, we suggest that SMC1L1 mutations have milder effects than NIPBL mutations with respect to pre- and postnatal growth retardation and associated malformations. If confirmed, these data may have important implications for directing mutation screening in CdLS.     

Confirmation of BRD4 haploinsufficiency role in Cornelia de Lange–like phenotype and delineation of a 19p13.12p13.11 gene contiguous syndrome

Cornelia de Lange syndrome (CdLS) is a genetically and clinical heterogeneous condition characterized by congenital malformation, intellectual disability, and peculiar dysmorphic features. Recently, BRD4 (19p13.12) was proposed as a new critical gene associated with a mild CdLS because of a similar presentation of the patients carrying point mutations and of its involvement in the NIPBL pathway. Patients harboring a 19p interstitial deletion shared some physical features with BRD4 mutation carriers, which results in a more complex phenotype because of the involvement of several neighboring genes. We report a new 19p deletion in a patient clinically diagnosed as CdLS, partially overlapping with previously published cases with the aim to support the role of BRD4 haploinsufficiency in a CdL‐like phenotype and to improve the delineation of 19p13.12p13.11 deletion as a new nonrecurrent gene contiguous syndrome, spanning GIPC1, NOTCH3, BRD4, AKAP8, AKAP8L, CASP14, and EPS15L1 genes. Previously described cases are reviewed, attempting to delineate a genotype–phenotype correlation.     

A homozygous variant in NDUFA8 is associated with developmental delay,microcephaly, and epilepsy due to mitochondrial complex I deficiency

Mitochondrial complex I deficiency is caused by pathogenic variants in mitochondrial and nuclear genes associated with complex I structure and assembly. We report the case of a patient with NDUFA8-related mitochondrial disease. The patient presented with developmental delay, microcephaly, and epilepsy. His fibroblasts showed apparent biochemical defects in mitochondrial complex I. Whole-exome sequencing revealed that the patient carried a homozygous variant in NDUFA8. His fibroblasts showed a reduction in the protein expression level of not only NDUFA8, but also the other complex I subunits, consistent with assembly defects. The enzyme activity of complex I and oxygen consumption rate were restored by reintroducing wild-typeNDUFA8 cDNA into patient fibroblasts. The functional properties of the variant in NDUFA8 were also investigated using NDUFA8 knockout cells expressing wild-type or mutated NDUFA8 cDNA. These experiments further supported the pathogenicity of the variant in complex I assembly. This is the first report describing that the loss of NDUFA8, which has not previously been associated with mitochondrial disease, causes severe defect in the assembly of mitochondrial complex I, leading to progressive neurological and developmental abnormalities.     

Variants in NGLY1 lead to intellectual disability,myoclonus epilepsy,sensorimotor axonal polyneuropathy and mitochondrial dysfunction

NGLY1 encodes the enzyme N-glycanase that is involved in the degradation of glycoproteins as part of the endoplasmatic reticulum-associated degradation pathway. Variants in this gene have been described to cause a multisystem disease characterized by neuromotor impairment, neuropathy, intellectual disability, and dysmorphic features. Here, we describe four patients with pathogenic variants in NGLY1. As the clinical features and laboratory results of the patients suggested a multisystem mitochondrial disease, a muscle biopsy had been performed. Biochemical analysis in muscle showed a strongly reduced ATP production rate in all patients, while individual OXPHOS enzyme activities varied from normal to reduced. No causative variants in any mitochondrial disease genes were found using mtDNA analysis and whole exome sequencing. In all four patients, variants in NGLY1 were identified, including two unreported variants (c.849T>G (p.(Cys283Trp)) and c.1067A>G (p.(Glu356Gly)). Western blot analysis of N-glycanase in muscle and fibroblasts showed a complete absence of N-glycanase. One patient showed a decreased basal and maximal oxygen consumption rates in fibroblasts. Mitochondrial morphofunction fibroblast analysis showed patient specific differences when compared to control cell lines. In conclusion, variants in NGLY1 affect mitochondrial energy metabolism which in turn might contribute to the clinical disease course.     

Mutation in mitochondrial complex IV subunit COX5A causes pulmonary arterial hypertension,lactic acidemia,and failure to thrive

COX5A is a nuclear‐encoded subunit of mitochondrial respiratory chain complex IV (cytochrome c oxidase). We present patients with a homozygous pathogenic variant in the COX5A gene. Clinical details of two affected siblings suffering from early‐onset pulmonary arterial hypertension, lactic acidemia, failure to thrive, and isolated complex IV deficiency are presented. We show that the variant lies within the evolutionarily conserved COX5A/COX4 interface domain, suggesting that it alters the interaction between these two subunits during complex IV biogenesis. In patient skin fibroblasts, the enzymatic activity and protein levels of complex IV and several of its subunits are reduced. Lentiviral complementation rescues complex IV deficiency. The monomeric COX1 assembly intermediate accumulates demonstrating a function of COX5A in complex IV biogenesis. A potential therapeutic lead is demonstrated by showing that copper supplementation leads to partial rescue of complex IV deficiency in patient fibroblasts.